144 Ho7varth : The Ice-borne Boulders of Yorkshire . 



areas which produce the Andesites, Ashes, Breccias, and kin- 

 dred rocks generally described as of the ' Borrowdale ' type. 



Those distributed along the east coast appear to have passed 

 out at Tees-mouth, carried by a glacier having free access to 

 the North Sea. Boulders of Shap Granite have been dredged 

 up many miles from the coast. 



Later, this free outlet was interrupted by ice advancing 

 across the North Sea in a south-westerly direction, and sweep- 

 ing down the coast of South Scotland, Northumberland, 

 Durham, and Yorkshire, in such force as to dam back the local 

 glaciers, and to distribute their then terminal moraines along 

 the east coast, mixing them with the Scandinavian rocks which 

 the invading ice carried. 



To the foreign ice-sheet the east coast cliffs acted as a 

 buffer, but its south-westerly trend enabled the coast line to 

 deflect it southwards, and the British ice which reached the 

 coast was dragged along with it. Where the coast line was 

 lower, or a river valley emerged, the foreign ice invaded the 

 land for shorter or longer distances as the surface elevation 

 (relative to the pressure of the ice-mass) controlled it, so that 

 in north-east Cleveland it either forced its mass or some of its 

 contents many miles inland, and again at Scarborough and in 

 Holderness. 



Indentations in the coastline, protected by cliffs with their 

 curves turned north-eastwards, acted as catchment basins for 

 erratics, so that they are more plentiful in such places as Robin 

 Hood's Bay, Scarborough, Speeton, Gristhorpe, the north side 

 of Flamborough, etc. In such localities they occur in thou- 

 sands. In many places, the ice topped the chffs, depositing 

 boulder clays, while in others it failed to do so. 



The blocking of the mouth of the Tees compelled the Stain- 

 moor and Teesdale glacier to leave its old course, and to turn 

 down the Vale of York, carrying with it its burden of Lake 

 Country rocks. These are traceable all down the Vale of York 

 as far as Escrick.* They are also found at Doncaster, Tickhill 

 and Bawtry. 



A line drawn from about Workington, on the coast of Cum- 

 berland, by the southern watershed of Thirlmere, and round 

 to the west side of Wastdale Crag in Westmorland, would appear 

 to mark the boundary line or * boulder-shed ' by which the 



* Kendall, Proc. Yorks. Geol. Soc, Vol. XII., part 4, 1893. 



Naturalist, 



